Andrew's git - gitweb.git/blob

   1#include "cache.h"
   2#include "levenshtein.h"
   3
   4/*
   5 * This function implements the Damerau-Levenshtein algorithm to
   6 * calculate a distance between strings.
   7 *
   8 * Basically, it says how many letters need to be swapped, substituted,
   9 * deleted from, or added to string1, at least, to get string2.
  10 *
  11 * The idea is to build a distance matrix for the substrings of both
  12 * strings.  To avoid a large space complexity, only the last three rows
  13 * are kept in memory (if swaps had the same or higher cost as one deletion
  14 * plus one insertion, only two rows would be needed).
  15 *
  16 * At any stage, "i + 1" denotes the length of the current substring of
  17 * string1 that the distance is calculated for.
  18 *
  19 * row2 holds the current row, row1 the previous row (i.e. for the substring
  20 * of string1 of length "i"), and row0 the row before that.
  21 *
  22 * In other words, at the start of the big loop, row2[j + 1] contains the
  23 * Damerau-Levenshtein distance between the substring of string1 of length
  24 * "i" and the substring of string2 of length "j + 1".
  25 *
  26 * All the big loop does is determine the partial minimum-cost paths.
  27 *
  28 * It does so by calculating the costs of the path ending in characters
  29 * i (in string1) and j (in string2), respectively, given that the last
  30 * operation is a substitution, a swap, a deletion, or an insertion.
  31 *
  32 * This implementation allows the costs to be weighted:
  33 *
  34 * - w (as in "sWap")
  35 * - s (as in "Substitution")
  36 * - a (for insertion, AKA "Add")
  37 * - d (as in "Deletion")
  38 *
  39 * Note that this algorithm calculates a distance _iff_ d == a.
  40 */
  41int levenshtein(const char *string1, const char *string2,
  42                int w, int s, int a, int d)
  43{
  44        int len1 = strlen(string1), len2 = strlen(string2);
  45        int *row0 = xmalloc(sizeof(int) * (len2 + 1));
  46        int *row1 = xmalloc(sizeof(int) * (len2 + 1));
  47        int *row2 = xmalloc(sizeof(int) * (len2 + 1));
  48        int i, j;
  49
  50        for (j = 0; j <= len2; j++)
  51                row1[j] = j * a;
  52        for (i = 0; i < len1; i++) {
  53                int *dummy;
  54
  55                row2[0] = (i + 1) * d;
  56                for (j = 0; j < len2; j++) {
  57                        /* substitution */
  58                        row2[j + 1] = row1[j] + s * (string1[i] != string2[j]);
  59                        /* swap */
  60                        if (i > 0 && j > 0 && string1[i - 1] == string2[j] &&
  61                                        string1[i] == string2[j - 1] &&
  62                                        row2[j + 1] > row0[j - 1] + w)
  63                                row2[j + 1] = row0[j - 1] + w;
  64                        /* deletion */
  65                        if (row2[j + 1] > row1[j + 1] + d)
  66                                row2[j + 1] = row1[j + 1] + d;
  67                        /* insertion */
  68                        if (row2[j + 1] > row2[j] + a)
  69                                row2[j + 1] = row2[j] + a;
  70                }
  71
  72                dummy = row0;
  73                row0 = row1;
  74                row1 = row2;
  75                row2 = dummy;
  76        }
  77
  78        i = row1[len2];
  79        free(row0);
  80        free(row1);
  81        free(row2);
  82
  83        return i;
  84}